1. Field of the Invention
The present invention relates generally to ballast circuits, and more particularly, to dimming ballast circuits used to selectively adjust the light output of flourescent lamps to provide a dimming range that can extend down to and below 10% of total lamp output.
2. Background Information
Dimming ballast circuits generally fall into one of two categories: isolated output dimming ballast circuits such as that of FIG. 1; and non-isolated output dimming ballast circuits such as that of FIG. 2. Dimming is achieved by controlling the frequency or pulse width of the current supplied to the lamps. For example, frequencies are controlled over a range of, for example, 20 kiloHertz (kHz) to approximately 100 kHz, with full lamp output being provided at approximately 20 kHz.
Because of interference with other systems, such as remote control devices, which can be caused in and around the 20 kHz portion of the frequency bandwidth, the frequency of the current is typically controlled within a range of 40 to 120 kHz. At these high frequencies, current can leak to ground via air due to a capacitive effect. The result is that the lamp ballast cannot produce a dimming of the lamp below 10%.
In addition, multiple lamps are often configured in series. To balance illumination between the lamps, it is necessary to accurately measure lamp current through the series connected lamps. However, the series configuration of the lamps includes interwiring and interwinding capacitance, which precludes accurate current measurements.
FIG. 1 shows an example of an isolated dimming ballast circuit 102 associated with multiple lamps 104 and 106. Isolated output dimming ballast circuits use a magnetic coupling component, such as galvanic separation, to sense the load current of lamps driven by the ballast circuit. Accordingly, the dimming ballast circuit 102 includes a current regulation control circuit 108 connected to a power switch (not shown) and a sensing circuit 110. The control circuit 108 includes a resonant inductor 112 in series with the primary winding 114 of a transformer 118 having a secondary winding 116 coupled in series with the lamps 104 and 106. A resonant capacitor 120 is connected in parallel with the primary winding 114. The sensing circuit 110 includes a Schottky diode 122 connected in series with the secondary winding 124 of a current sensing transformer 126 having a primary winding 128 connected in series with the lamps 104 and 106. A resistor R1 labeled 130 is connected in series with the secondary winding 124.
Because the isolated ballast circuit of FIG. 1 is subject to high interwinding and interwiring capacitance, multiple lamp configurations cannot be dimmed below 10%, such that the current sensing loses accuracy. For example, interwinding capacitances such as Cwdg exist between the primary and secondary windings of each of the transformers 118 and 126. Interwiring capacitances such as Cwdg exist among the various conductors of the ballast circuit. In addition, parasitic capacitances such as Cp exist in the air due to leakage from the lamps 104 and 106 to ground. The value of these capacitances can vary with frequencies with, for example, increased capacitance being associated with increased frequency. These capacitances create current leakage paths.
The effect of current leakage in a dimming ballast circuit due to parasitic capacitances at high lamp operating frequencies can be demonstrated by the following: assume the total lamp current through the lamps 104 and 106 is 500 milliamps (mA) at full (i.e., 100%) light output, with the loss at the lamp 106 being 2 mA and the loss at lamp 104 being 1 mA (i.e., 497 mA actually passes through the lamps 104 and 106). At 10% of the total light output, where frequency of the supply current has been increased, assume total current through the lamps is 50 mA, with the loss at lamp 106 being approximately 4 mA, and the loss at lamp 104 being approximately 2 mA (i.e., 44 mA actually passes through the lamps). The differing losses at each of the two lamps will not produce a noticeable difference in light output from the two lamps at 100% light output. However, at 10% of total light output, a noticeable difference in the light output will be produced from each of the lamps (i.e., an imbalance in lamp output).
An additional problem occurs when an attempt is made to further dim the lamp below 10%. For example, at 1% of total light output, where frequency has been increased even further, assume that total lamp current is 5 mA. The loss at lamp 106 will increase above the 4 mA loss experienced at 10% of total output, and the loss at lamp 104 will increase above the 2 mA. Thus, at 1% of total light output, the lamp losses exceed the total available current, such that the lamps are extinguished and dimming cannot even be achieved.
FIG. 2 shows an exemplary non-isolated dimming ballast circuit 202 associated with multiple lamps 204 and 206 connected in parallel. The ballast circuit 202 includes a current regulation control circuit having parallel resonant inductors 208 and 210 connected between a power switch and each of the lamps 204 and 206, respectively. Resonant capacitors 212 and 214 are connected in parallel with each of the lamps 204 and 206, respectively. A sensing circuit 218 of the dimming ballast includes a first resistor 220 and a second resistor 222 connected to each of the lamps 204 and 206, respectively for providing current measurement.
The use of a parallel lamp configuration as shown in FIG. 2 avoids some of the detrimental effects due to interwiring and interwinding capacitance. However, parasitic capacitances can result in current imbalances and current losses which produce effects similar to those described with respect to FIG. 1. In addition, when non-isolated dimming ballast circuits as shown in FIG. 2 are used, a non-common wiring (such as an 8-wire configuration) is used versus US standard ANSI wiring (6 wires). That is, where multiple lamps are used in a non-isolated dimming ballast circuit, 8 wires are used to connect all cathodes and to separately sense current in each lamp independently so that current imbalances can be compensated.
Accordingly, it would be desirable to provide a dimming ballast circuit which can accurately sense and control current in each of multiple lamps independently to permit dimming over an entire range of total lamp output, down to and below 10% of total lamp output, in such a manner that the lamp outputs remain balanced. It would also be desirable to provide such a capability using standard US wiring configurations.
The present invention is directed to accurately sensing and regulating lamp current, and thus lamp output, over a range of lamp outputs, down to and below 10% of total lamp output. Exemplary embodiments can be used with a single set of power elements to drive multiple lamps in either an isolated or a non-isolated condition.
Generally speaking, exemplary embodiments are directed to a method and apparatus for controlling a ballast circuit comprising means for sensing current at each of multiple outputs of a ballast circuit configured to supply current to each lamp to be driven; and means for balancing current among each of said multiple outputs.